The present invention relates to a non-contact type hydrodynamic gas-bearing in which hydrodynamic pressure is utilized.
For the purpose of increasing data transmission speed of a recording apparatus such as a hard disc apparatus in which a rotating disc is employed, it is required to increase the rotating speed of a motor for driving the disc, so that the disc will rotate at a higher speed. In such a motor designed for high-speed rotation, a non-contact type hydrodynamic gas bearing is employed.
Examples of conventional hydrodynamic gas bearing will be described referring to FIG. 3 and FIG. 4.
FIG. 3 is a cross-sectional view of a hydrodynamic gas bearing according to a first prior art. Referring to FIG. 3, a supporting shaft 12 that is a fixed member is inserted into a bearing bore 13b of a cylindrical hub 13 that is a rotatable member. The supporting shaft 12 is provided with a number of hydrodynamic pressure generating grooves 12b of zigzag pattern formed at regular intervals all over its outer circumferential surface 12a. In FIG. 3, it is to be understood that the illustration of the supporting shaft 12 is actually a schematic side view instead of a sectional view. The schematic side view is for showing that the hydrodynamic pressure generating grooves 12b are formed at regular intervals, rather than exactly showing the supporting shaft 12 seen from a side. The side view of the supporting shaft 12 shows in schematical manner in the manner used in relevant technical field when showing a number of hydrodynamic pressure generating grooves 12b formed at regular intervals on the outer circumferential surface 12a of the supporting shaft 12. Therefore it does not represent the real appearance in draftsmanship. The supporting shaft 12 is provided with a projection 12c on its lower face in the drawing. The projection 12c is fixed to a thrust flange 14a having a flange portion 14. The thrust flange 14a is fixed to a frame 11. On the frame 11, stator 18 is mounted.
The hub 13 is provided with a cylindrical rotor attaching base 13a at its lower portion In the drawing. On the inner wall of the rotor attaching base 13a, a rotor 19 is attached in opposed relationship to the stator 18. On the rotor attaching base 13a, an annular thrust bearing plate 5 is mounted so as to locate between the lower end face 12d of the supporting shaft 12 and the flange portion 14. The upper face 5a and the lower face 5b of the thrust bearing plate 5 are respectively provided with hydrodynamic pressure generating grooves 25a and 25b, as shown in FIG. 5 and FIG. 6. In a static state where the hub 13 is not rotating, the lower face 5b of the thrust bearing plate 5 is in contact with the flange portion 14.
When an electric power is supplied to the stator 18, the rotor 19 generate a rotative force, so that the hub 13 starts to rotate in the direction of an arrow A (clockwise direction when the hub 13 is viewed from an upper direction of the drawing). When the rotating speed of the hub 13 reaches a predetermined value, for example 1000 rpm, air pressure in the clearance between the supporting shaft 12 and the hub 13 increases in the central region 2G of the supporting shaft 12, owing to an air compressing effect by the hydrodynamic pressure generating grooves 12b on the supporting shaft 12. The increase of air pressure allows the hub 13 to rotate without contacting with the supporting shaft 12, maintaining a clearance between the inner wall of the hub 13 and the surface of the supporting shaft 12.
Referring to the thrust bearing plate 5, air pressure increases owing to air compressing effect by the hydrodynamic pressure generating grooves 25a and 25b on the thrust bearing plate 5 between the upper face 5a of the thrust bearing plate 5 and the lower face 12d of the supporting shaft 12 in the vicinity of the projection shaft 12c. In a similar manner, air pressure increases between the lower face 5b of the thrust bearing plate 5 and the upper face 14b of the flange portion 14 in the vicinity of the projection 12c. The increase of air pressure between the upper face 5a of the thrust bearing plate 5 and the lower face 12d of the supporting shaft 12 generates a force to press the hub 13 downward. On the other hand, the increase of air pressure between the lower face 5b of the thrust bearing plate 5 and the upper face 14b of the flange portion 14 will generate a force to press the hub 13 upward. When the rotating speed of the hub 13 reaches a predetermined value and the force to press the hub 13 upward (hereafter referred to as “thrust force”) becomes great enough to support the weight of the hub 13, the hub 13 will be lifted. As a result the thrust bearing plate 5 is separated from the flange portion 14 and supported between the lower face 12d of the supporting shaft 12 and the upper face 14b of the flange portion 14 without contacting with either of them. In a state where the hub 13 is supported without a contact, a force to press the hub 13 upward is balanced with a total of the weight of the hub 13 and the force to press the hub 13 downward.
According to the first prior art, since gas is employed which has less viscosity than oil, air pressure between the lower face 5b of the thrust bearing plate 5 and the upper face 14b of the flange portion 14 may not be great enough until the rotating speed of the hub 13 reaches a considerable level. Therefore the thrust bearing plate 5 will rotate in contact with the upper face 14b of the flange portion 14, causing abrasion of the lower face 5b of the thrust bearing plate 5 and the upper face 14b of the flange portion 14 due to friction. Since the friction of the lower face 5b of the thrust bearing plate 5 and the upper face 14b of the flange portion 14 will take place each time the motor is activated, this prior art hydrodynamic gas bearing has a disadvantage that the life span is short due to the abrasion caused by the friction. Besides, fine chips produced from such abrasion are splashed around and stuck to a magnetic disc etc., and would resultantly impair the reliability of the apparatus.
A hydrodynamic gas bearing according to a second prior art, which was developed to solve the foregoing problems, shall now be described referring to a cross-sectional view of FIG. 4. As shown in FIG. 4, the supporting shaft 12 is inserted into the bearing bore 13b of the rotatable cylindrical hub 13. The supporting shaft 12 is provided with hydrodynamic pressure generating grooves 12b formed in zigzags all over its outer circumferential surface 12a The supporting shaft 12 is also provided with a projection 12f on its lower face in the drawing, and the projection 12f is fixed to the frame 11. On the frame 11, the stator 18 is mounted. On the inner wall of the rotor attaching base 13a at the lower portion of the hub 13, the rotor 19 is attached in mutually opposed and rotatable relationship to the stator. The supporting shaft 12 is provided with the other projection 12e on its upper face. An annular permanent magnet 16a is attached to the uppermost end portion of the projection 12e, with an attaching member 17 disposed therebetween. At the base of the projection 12e, an annular permanent magnet 16d is attached on the upper end face of the supporting shaft 12 so as to maintain a predetermined interval from the permanent magnet 16a. 
On a flange 13c provided at an upper portion of the hub 13 in the drawing, two annular permanent magnets 16b and 16c are concentrically attached so as to be located between the permanent magnets 16a and 16d. The permanent magnets 16a and 16b are magnetized to mutually generate a repulsive force. Also the permanent magnets 16c and 16d are magnetized to mutually generate a repulsive force.
In the hydrodynamic gas bearing according to the second prior art, the repulsive force between the permanent magnets 16a and 16b as well as that between 16c and 16d are supporting the weight of the hub 13 in the thrust direction (in a vertical direction in the drawing). Therefore the permanent magnets 16a and 16b are not in mutual contact, neither do the permanent magnets 16c and 16d, even while the hub 13 is not rotating. However, in case where a shaking force in the thrust direction is imposed to the hub 13 due to an external force, the hub 13 may largely move in the thrust direction, thus causing the permanent magnets 16a and 16b, or the permanent magnets 16c and 16d, to make mutual contacts. Due to such contacts that may occur while the hub 13 is rotating, permanent magnets that are relatively brittle may chip at a contact point or even break depending on the extent of impact.